1. Field of the Invention
The present invention pertains to a method for etching of patterned dielectric layers. The method is useful in etching particularly thick layers of dielectric materials, in the range of several thousand angstroms to several micrometers thick, and particularly in the etching of high aspect ratio vias or open trenches including self-aligned contact structures.
2. Brief Description of the Background Art
In the field of semiconductor device fabrication, particularly with the continuing trend toward smaller device feature sizes, the etch processes which are used to construct conductive features such a metal interconnects and contacts have become particularly critical. The new devices, having feature sizes in the range of about 0.18 xcexcm and smaller, place an emphasis on both the precise profile achievable during pattern etching. To obtain the smaller feature sizes, the imaging radiation used to provide the pattern has moved toward shorter and shorter wavelengths, with DUV photoresists replacing the commonly used I-line photoresists. Many of the commonly used photoresist materials are not sufficiently transparent for such short wave length radiation, and the thickness of the photoresist layer has had to be reduced to accommodate the imaging process. Since the etch rate of the photoresist materials relative to the etch rate of underlying layers to be etched has not significantly changed, it is not possible to etch thick underlying layers.
We have been working to develop an etching process which permits the development of patterning masks which can transfer a desired pattern to adjacent layers in a manner which reduces or avoids the formation of mask residue on the etched structure. Further, we have worked to develop an etching process useful in etching organic polymeric materials for damascene processes, where a conductive layer is applied over the patterned surface of a dielectric layer to form desired conductive interconnect and contact structures. We now apply the concepts which we have developed to provide a method of etching particularly thick dielectric layers.
FIGS. 1A-1E show a schematic cross-sectional view of a typical plasma etch stack for etching a metal-comprising layer at temperatures in excess of about 150xc2x0 C. as it progresses through a series of steps including the etching of both dielectric and conductive layers. This etch stack is of the kind known and used prior to application of the concepts of which are subsequently described herein both for purposes of etching both conductive and dielectric layers.
FIG. 1A shows a complete etch stack, including, from bottom to top, substrate 102, which is typically a dielectric layer overlying a semiconductor substrate (such as a silicon wafer substrate) or which may be the semiconductor material itself, depending on the location on a given device surface. Barrier layer 104, which prevents the diffusion and/or migration of material between conductive layer 106 and substrate 102; conductive layer 106, which is typically aluminum or copper, but might be tungsten, platinum, or iridium, for example. Anti-reflective-coating (ARC) layer 108, which is typically a metal-containing compound and which enables better imaging of an overlying patterning layer. Pattern masking layer 110, which is typically a layer of silicon dioxide or similar inorganic material which can withstand the high temperatures encountered during etching of conductive layer 106, and which can be patterned and used as a mask during such etching. And, photoresist layer 112 which is typically an organic-based material which is stable at low temperatures and which is used to pattern masking layer 110, which is stable at higher temperatures. In FIG. 1A, photoresist layer 112 has already been patterned to provide the feature shape desired to be transferred to pattern masking layer 110.
FIG. 1B shows the stack described in FIG. 1A, where the pattern in photoresist layer 112 has been transferred to pattern masking layer 110, using a standard plasma etching technique. When masking layer 110 comprises a silicon-containing material, such as silicon dioxide, the etch plasma typically comprises a fluorine-generating species. Preferably, the plasma selectivity is for the silicon dioxide over the photoresist material.
FIG. 1C shows the next step in the process of etching conductive layer 106, where the photoresist layer 112 has been stripped from the surface of pattern masking layer 110. This stripping procedure may be a wet chemical removal or may be a plasma etch which is selective for the photoresist layer 112 over the pattern masking layer 110. Stripping of photoresist layer 112 is carried out for two reasons. The organic-based photoresist materials typically used for layer 112 would melt or become distorted in shape at the temperatures commonly reached during the etching of conductive layer 106. This could lead to distortion of the pattern which is transferred to conductive layer 106. In addition, polymeric species generated due to the exposure of the surface of photoresist layer 112 to the etchant plasma tend to contaminate adjacent surfaces during the etching of conductive layer 106, thereby decreasing the etch rate of conductive layer 106.
The procedure of using a photoresist material to pattern an underlying silicon oxide patterning layer is described in U.S. Pat. No. 5,067,002 to Zdebel et al., issued Nov. 19, 1991. Zdebel et al. mention the need to remove the photoresist material prior to etching of underlying layers, to avoid contamination of underlying surfaces with the photoresist material during etching of such underlying layers. David Keller describes the use of an ozone plasma for the purpose of dry etch removal of a photoresist mask from the surface of an oxide hard mask in U.S. Pat. No. 5,346,586, issued Sep. 13, 1994. Mr. Keller also mentions that it is easier to etch selectively to a gate oxide when there is no photoresist present during a polysilicon gate oxide etch step.
FIG. 1D shows the next step in the etching process, where the desired pattern has been transferred through ARC layer 108, conductive layer 106, and barrier layer 104. Typically, all of these layers are metal comprising layers, and a halogen containing plasma can be used to etch the pattern through all three layers. At this point, the problem is the removal of the residual silicon dioxide hard masking material and the removal of residue deposits of the silicon dioxide masking material from adjacent surfaces. The residual hard masking material is present as residual masking layer 110, and the residue deposits as 114 on the surface of the patterned conductive layer 106 and the surface of substrate 102.
In the case of the deposit 114 on the surface of patterned conductive layer 106, deposit 114 can trap residual chemical etch reactants under deposit 114 and against the surface of patterned conductive layer 106, leading to subsequent corrosion of conductive layer 106. That corrosion is shown on FIG. 1D as 116.
In addition, when substrate 102 is a low dielectric constant material, for purposes of reducing electrical interconnect delays, residual masking layer 110 which remains after pattern etching through layers 108, 106, and 104 (as shown in FIG. 1D) can deteriorate device performance or cause difficulties in future processing steps (particularly during contact via etch). This makes it important to remove any residual masking layer 110 from the surface of ARC layer 108.
Further, when a dielectric layer 118 is applied over the surface of the patterned conductive layer 106, as shown in FIG. 1E, if residual masking layer 110 is not removed, a non-planar surface 120 is produced. A non-planar surface creates a number of problems in construction of a multi-conductive-layered device, where additional patterned conductive layers (not shown) are constructed over the surface 120 of dielectric layer 118.
With the above considerations in mind, we wanted to develop a patterning system, including a multi-layered structure and a method for its use which would provide for the easy removal of residual masking layer material after completion of the patterning process.
We subsequently discovered that the patterning method developed to provide for easy removal of residual masking layer material is particularly useful in the etching of thick (typically greater than 1,000 xc3x85) layers of dielectric materials, to provide self-aligned contact structures having a high aspect ratio (typically greater than about 5:1, and commonly as high as 15:1) and open trenches including self-aligned contact structures.
A first embodiment of the present invention pertains to a method of patterning semiconductor device features and conductive features in general, wherein the method provides for the easy removal of any residual masking layer which remains after completion of a pattern etching process. The method provides for a multi-layered masking structure which includes a layer of high-temperature organic-based masking material overlaid by either a layer of a high-temperature inorganic masking material (such as silicon oxide, silicon nitride, or silicon carbide) which can be patterned to provide an inorganic hard mask, or by a layer of high-temperature imageable organic masking material, such as PPMS, which can be processed and patterned to provide a hard mask. The hard masking material is used to transfer a pattern to the high-temperature organic-based masking material, and then the hard masking material is removed. The high-temperature organic-based (carbon-based) masking material is used to transfer the pattern to an underlying semiconductor device feature. The high-temperature organic-based masking material can be removed from the surface of the patterned semiconductor device feature in a manner which reduces or avoids contamination of the patterned feature surface.
In accordance with embodiments of the present invention, we have developed two patterning systems which enable the patterning of underlying layers at relatively high temperatures, ranging between about 150xc2x0 C. and about 500xc2x0 C., while providing easy removal of any residual masking layer remaining after the patterning process.
The first patterning system uses a multi-layered masking structure which includes a layer of high-temperature organic-based masking material overlaid by a layer of a high-temperature inorganic masking material, which is further overlaid by a layer of a patterning photoresist.
The patterning method is as follows.
a) The layer of photoresist material is imaged and developed into a pattern using techniques known in the art, to produce a patterned mask which can be used to transfer the desired pattern throughout the multi-layered masking structure and eventually through at least one device feature layer as well.
b) The patterned photoresist is used to transfer the pattern through
i) a layer of high-temperature inorganic masking material; and
ii) a layer of high-temperature organic-based masking material.
Preferably, the pattern transfer through the layer of high-temperature organic-based masking material is via an anisotropic plasma etch technique so that this material is not undercut by the pattern transfer process.
c) Residual photoresist which remains after pattern transfer is then removed from the multilayered structure by plasma etch, using the high-temperature inorganic masking layer as an etch stop. The photoresist removal is accomplished using an anisotropic etch process which typically comprises an oxygen-based plasma etch. The anisotropic stripping of the photoresist prevents or at least substantially reduces any etching of the high-temperature organic-based masking material during photoresist removal.
d) Optionally, the layer of high temperature inorganic masking material may be removed at this time using a plasma etch technique or a wet etch technique designed to minimize any etching of the organic-based masking material. Preferably, the high temperature inorganic masking material is of a thickness such that it will be automatically removed during etching of a feature layer (step e).
e) The pattern is then transferred from the high-temperature organic-based masking layer through at least one feature layer underlying the high-temperature organic-based masking material.
f) Any high-temperature organic-based masking material remaining after feature layer patterning is then easily removed using a plasma etch technique. When the etched feature layer would be corroded or oxidized by an oxygen-based plasma, a hydrogen/nitrogen-based plasma etch technique is recommended. The removal of organic-based masking material may be by a wet stripping technique using a solvent known in the art to be advantageous in the passivation of the surface of the patterned feature layer.
Since there is no residual photoresist material remaining from step a) present during etching of the feature layer, there is no layer which is likely to melt or distort in shape during transfer of the pattern from the high-temperature organic-based masking material to an underlying device feature layer.
Since the high-temperature organic-based masking layer is easily removed, there need be no residual masking layer present in the device structure to affect device performance or to cause difficulties during subsequent via etch. Preferably, the high-temperature organic-based masking layer is formed from xcex1-C and xcex1-FC films deposited using CVD techniques. Examples of starting materials used to form such films include CH4, C2H2, CF4, C2F6, C4F8, NF3, and combinations thereof; there are, of course, numbers of other carbon-containing precursor materials which can also be used. The starting materials which contain less (or no) fluorine are preferred.
The second patterning system is different from the first patterning system in that it uses a high-temperature pattern-imaging layer rather than a more standard photoresist imaging layer. The high-temperature pattern-imaging layer is stable at temperatures ranging from about 150xc2x0 C. to about 500xc2x0 C., compared with photoresist materials which are generally stable at about 150xc2x0 C. or lower. Preferably, the high-temperature pattern-imaging layer is a plasma-polymerized material; more preferably, the pattern-imaging layer is an organo-silicon compound, such as plasma polymerized methyl silane (PPMS), which may be imaged by deep UV and which is plasma-developable. However, the high-temperature imaging layer may also be formed from a different silane-based starting material such as a TEOS-based (tetra-ethyl-ortho-silicate-based) chemistry, and one skilled in the art may select from other similar materials known in the art.
The patterning method is as follows.
a) A layer of high-temperature imageable material is imaged and developed into a pattern using techniques known in the art, to produce a patterned mask which can be used to transfer the desired pattern through the high-temperature organic-based masking material and eventually through at least one device feature layer.
b) After patterning of the high-temperature imageable material, the pattern is transferred through the underlying layer of high-temperature organic-based masking material. Preferably, the pattern is transferred via an anisotropic etch technique, whereby the high-temperature organic-based masking material is not undercut by the pattern transfer step.
c) The pattern is then transferred from the multi-layered structure formed in steps a) and b) through at least one feature layer underlying the high-temperature organic-based masking material. Preferably, the pattern is transferred using an anisotropic etching technique so that any high-temperature imageable material which might remain from step b) is removed during this pattern transfer step. In addition, the use of an anisotropic etching technique reduces or avoids the possibility of undercutting the high-temperature organic-based material layer during the pattern transfer to the underlying device feature layer.
d) Any residual high-temperature organic-based masking material which remains after pattern transfer is then easily removed using a plasma etch technique. When the etched feature layer would be corroded or oxidized by an oxygen-based plasma, a hydrogen-based plasma etch technique is recommended.
Since there is no low temperature residual photoresist material used during this process, there is no layer which is likely to melt or distort in shape during transfer of the pattern from the high-temperature organic masking material to an underlying device feature layer.
As previously mentioned, the high-temperature imageable material is preferably a silane-based starting material such as plasma polymerized methyl silane (PPMS), or a TEOS-based (tetra-ethyl-ortho-silicate-based) material, and one skilled in the art may select from other similar materials known in the art.
The high-temperature organic-based masking material is preferably chosen from materials which can be easily removed by plasma etch techniques or by using a solvent known in the art to be advantageous in the passivation of the surface of the patterned feature layer. Examples of such materials are provided above with reference to the first patterning system.
When the device feature layer which is to be patterned includes a copper layer, that copper layer is preferably pattern etched using either an enhanced physical bombardment technique or a plasma etching technique which generates sufficient hydrogen to protect the copper surface during patterning.
The above-described method is also useful in the pattern etching of aluminum or tungsten, even though these metals can be etched at lower temperatures, typically below about 200xc2x0 C. (because the etch reaction byproducts are more volatile). When the feature size of an aluminum-comprising structure is about 0.25 xcexcm and smaller, use of a hard mask patterning layer, such as silicon dioxide, silicon nitride, or silicon carbide provides a sufficiently long mask lifetime during etching of an underlying aluminum-comprising layer. Further, since the selectivity toward aluminum is better when such a hard mask patterning layer is used rather than a typical organic-based photoresist, the masking layer thickness can be less, the aspect ratio is lower, and the possibility of shading damage to the device structure is reduced. The present method makes removal of the hard masking material possible without damage to the substrate, which is advantageous in subsequent processing.
The above-described method is also useful in the pattern etching of low k dielectric materials. In instances when the high-temperature organic-based masking material is used as the principal masking material (acts as the hardmask for pattern etching an underlying layer of a low k dielectric material, whether the dielectric material is an inorganic material or an organic material), it is important to select the high-temperature organic-based masking material so that it etches sufficiently more slowly than the underlying low k dielectric material, i.e. the selectivity for the particular plasma source gas composition being used to etch the underlying low k dielectric material favors etching of the underlying low k dielectric material relative to the high temperature organic-based masking material. Selectivity toward etching the low k dielectric material relative to the organic-based masking material is preferably substantially greater than 1.
A second embodiment of the present invention pertains to a specialized etch chemistry useful in the patterning of organic polymeric layers such as low dielectric constant (low k dielectric) materials and other organic polymeric interfacial layers. This etch chemistry is especially useful in a multilayered substrate of the kind described above. It is also useful in etching damascene structures where a metal fill layer is applied over the surface of a patterned organic-based dielectric layer.
In particular, when the organic, polymeric material being etched serves as a patterning mask for etch of a copper film, or when the organic, polymeric-material being etched is part of a damascene structure which is to be filled with copper, the etch chemistry provides for use of etchant plasma species where the oxygen, fluorine, chlorine, and bromine content is minimized. Preferably, essentially none of the plasma source gas material furnishes reactive species comprising oxygen, fluorine, chlorine, or bromine. When a patterning mask for etch of a copper film is prepared, should these etchant species contact the copper film, the film is oxidized or otherwise corroded. In addition, etchant species which contain oxygen, fluorine, chlorine or bromine leave deposits of etch byproduct on etched contact via and damascene structure surfaces which may cause oxidation and corrosion of copper depositions made over such surfaces. Further, during etching of some organic-comprising materials, oxygen and fluorine etchant species tend to have a detrimental effect on a typical contact via or trench etch profile.
When the conductive material is aluminum, tungsten, platinum, or iridium, rather than copper, the presence of oxygen is more acceptable during etch of the organic polymeric material. When the metal fill layer is tungsten, platinum, or iridium, the effect of the presence of fluorine, chlorine and bromine depends on the particular material used, as is known to one skilled in the art. However, even when the conductive material is aluminum, tungsten, platinum or iridium, oxygen-comprising or halogen etchant species are typically used as additives to increase etch rate or improve etch profile, or to control residue on an etch surface, but are not the principal etchant species for etching of the organic, polymeric material.
When an organic, polymeric material serves as a patterning xe2x80x9chardmaskxe2x80x9d for etch of an underlying dielectric material (where the dielectric material has a thickness greater than 1,000 xc3x85, and typically greater than 5,000 xc3x85), selection of the organic polymeric hard masking material and the plasma source gas used for etching of the dielectric material must be carefully matched. The combination of hard masking material and plasma source gas must provide the necessary high selectivity, typically greater than 3:1, where the etch rate of the dielectric material is at least 3 times greater than the etch rate of the organic polymeric masking material.
When the underlying dielectric material is inorganic, for example silicon oxide, silicon nitride, silicon oxynitride, doped silicon oxide, SOG, BPSG, and similar materials, the source gas used to etch an organic, polymeric masking material (underlying a hardmask or which is acting as a hard masking material) typically comprises oxygen, hydrogen, ammonia, and other active species which react with carbon to form volatile byproducts. The source gas used during etching of the underlying inorganic dielectric layer may comprise a reactive etchant hydrofluorocarbon such as CxHyFz. A preferred plasma source gas includes CHF3, which may be used in combination with additives such as CO, O2, N2, Ar, and other materials known in the art for etching inorganic dielectric layers. Other reactive etchants known in the art for etching silicon-containing layers may also be used, and the present example etchants are not intended to be limiting. The important considerations in selecting the plasma source gas for etching the inorganic dielectric layer are the etch selectivity of at least 3 for etching the inorganic dielectric layer in preference to the organic, polymeric masking layer, with the etch rate of the inorganic dielectric layer being at least 3,000 xc3x85 per minute, so that the process is economically competitive.
When the underlying dielectric layer is an organic material, such as one of the low k dielectric materials, then the overlying organic masking layer must be one which has a sufficiently different chemical and/or structural composition to provide the necessary selectivity between the low k dielectric material etch and the overlying organic masking layer; preferably the masking layer is a high temperature, organic-based material.
One preferred etch plasma for etching a polymeric, organic masking layer is a hydrogen/nitrogen-based plasma, wherein the principal etchant species is hydrogen, or nitrogen, or a combination thereof. In addition, as described above, depending on the underlying layer to be etched, the concentration of at least one etchant species such as oxygen, chlorine, fluorine, and bromine is carefully controlled. To provide a hydrogen/nitrogen-based plasma, the plasma etchant species comprise principally hydrogen, or principally nitrogen, or principally a mixture thereof. To provide these species, the plasma source gas comprises at least one of the materials selected from the group consisting of hydrogen, nitrogen, ammonia and compounds thereof, hydrazine and compounds thereof, hydroazoic acid, and combinations thereof. When an underlying conductive layer is to be etched, and that conductive layer is not copper, but is instead aluminum, tungsten, platinum, or iridium, the plasma source gas may comprise hydroxylamine or a compound thereof. One preferred plasma source gas for etching an organic, polymeric masking material (when the underlying layer to be pattern etched is not copper and is not an organic low k dielectric) comprises ammonia; or hydrogen and nitrogen; or a combination of ammonia with hydrogen, nitrogen, or both.
Other gases which provide essentially non-reactive etchant species, such as argon, helium, neon, krypton, and xenon may be present in varying amounts, to provide surface ion bombardment or to function as a diluent, by way of example, and not by way of limitation.
In addition, the addition of a small amount of hydrocarbon may be beneficial during etching of the high temperature organic polymeric material, to provide etch profile control.
It is also possible to use a hydrocarbon-based plasma for etching of the high temperature organic polymeric material. The hydrocarbon-based plasma may optionally include a lesser amount of a component selected from the group consisting of ammonia, hydrogen, nitrogen, and combinations thereof. Other gases which provide essentially non-reactive etchant species, such as argon, helium, neon, krypton, and xenon may also be present in varying amounts, by way of example.
When an integrated series of process steps is carried out in a single process chamber and a process step produces a byproduct which includes at least one element harmful to the final device structure, wherein that element is selected from the group consisting of oxygen, fluorine, chlorine, or bromine, it is advisable to dry clean the process chamber subsequent to that process step and prior to proceeding to an etch step using the present method for etching an organic polymeric material. This is particularly important when the feature size of the pattern being etched is 0.25 xcexcm or smaller. As an alternative to repetitive cleaning of process chambers, it is possible to use an integrated processing system which provides several processing chambers which are interconnected in a manner such that a substrate can be passed from chamber to chamber under a controlled environment, and to reserve one of such process chambers for etching using the present method etch chemistry.
An economical method of performing the etch techniques described above utilizes a combination of different plasmas wherein the different etchant gases used to create each plasma are sufficiently compatible that all of the etching steps can be carried out in individual (separate) steps in the same etch chamber, if desired. One skilled in the art can select from the various known plasma etchants to obtain the best economies of function which will provide etched features meeting dimensional and surface stability requirements.